PTFE fine powder prepared by coagulating and drying an aqueous PTFE dispersion is used as a coating for wire and cable, a tube having a small or large diameter, sealing tape, etc. In such applications of PTFE fine powder, the relationship between the characteristics of PTFE fine powder and its processability has been extensively studied and it has been found, for example, that PTFE fine powder having a larger average particle size is paste extruded at a lower extrusion pressure.
Various processes for preparing an aqueous PTFE dispersion comprising colloidal PTFE particles having a comparatively large average particle size have been developed and described, for example, in Japanese Patent Publication Nos. 16154/1968, 39829/1970 and 14466/1971.
The process described in Japanese Patent Publication No. 16154/1968 is characterized in that, in polymerization of tetrafluoroethylene (hereinafter referred to as "TFE") in an aqueous medium, 1.times.10.sup.-5 to 1.times.10.sup.-4 mol/l of a water soluble compound selected from the group consisting of water soluble compounds of zinc, aluminum and alkaline earth metals which afford cations when dissolved in the aqueous medium is added to the aqueous medium. Thereby, the average particle size of the PTFE particle is controlled. The metal ion to be added in this process, however, finally constitutes a contaminant in the polymerization system and reduces interfacial potential of the colloidal particle resulting in deterioration of stability of the system.
The processes described in Japanese Patent Publication Nos. 39829/1970 and 14466/1971 are characterized in that the average particle size of PTFE is controlled by the addition of a specific dispersant (i.e. a surfactant) at a specific time in the course of the polymerization. Although mono-dispersed collidal PTFE particles having a large average particle size are prepared by these processes, it is not convenient to add an aqueous concentrate of the dispersant in the course of the polymerization.